1. Technical Field
The present invention relates to printers of facsimile machines, copying machines, and OA equipment, and in particular, it relates to a what-is-called inkjet printer that ejects fine particles of multicolor liquid ink onto print paper (recording material) to draw predetermined characters and images, a program and a method for printing, an image processor and a program and a method for image processing, and a recording medium in which the programs are stored.
2. Related Art
Printers that employ an inkjet system (hereinafter, referred to as inkjet printers) generally draw characters or images onto a print medium (paper) to form a desired print by ejecting liquid-ink particles in dot form from the nozzles of a print head while moving a moving body called a carriage including a combination of an ink cartridge and the print head from side to side relative to the paper feed direction on the print paper. Since the carriage is equipped with ink cartridges of four colors (yellow, magenta, cyan, and black) and print heads for the individual colors, not only monochrome printing but also full-color printing of a combination of the four colors are facilitated (in addition to the four colors, six, seven, and eight colors including light cyan, light magenta, and so are in practical use).
With this type of inkjet printers that perform printing while moving the print head on a carriage laterally relative to the paper feed direction (across the width of print paper), the print head must be reciprocated from several tens to 100 times or more to print the whole page completely. This poses the problem of taking extremely more printing time than printers of other systems, e.g., electrophotographic laser printers such as copying machines.
In contrast, with inkjet printers of the type that has a print head with the same length as the width of print paper without a carriage, there is no need to move the print head along the width of the print paper, allowing what-is-called one-pass printing, which allows high-speed printing as with the laser printers. Also, there is no need to have a carriage for a print head and a driving system for moving the carriage. This offers the advantages that more compact and lightweight printer casing can be achieved, and more silent printers can be provided. The former inkjet printers are called “multipass printers”; the latter inkjet printers are called “line-head printers).
The print heads necessary for such inkjet printers have fine nozzles having diameters from about 10 to 70 μm arranged in series or in multistage in the printing direction at regular intervals. This may cause a so-called “droplet deflection phenomenon” that the direction of ink ejection of part of the nozzles tilts or the nozzles are displaced from an ideal position because of production error and as such, the dots formed by the nozzles deviate from targets.
As a result, poor printing that is a so-called “banding phenomenon” occurs in the part corresponding to the failed nozzle to decrease the print quality seriously. More specifically, when “the droplet deflection phenomenon” occurs, the distance between adjacent dots becomes uneven to cause “white lines (for white print paper)” in the part where the distance between adjacent dots is long, and “dark lines” in the part where the distance between adjacent dots is short.
Specifically, the banding phenomena tends to appear with the “line-head printers” that have a fixed print head (one-pass printing) and having a markedly larger number of nozzles than with the “multipass printers” (the multipass printers can make white lines inconspicuous by using the reciprocating motion of the print head).
Accordingly, to prevent the poor printing due to the “banding phenomenon”, research and development of hardware aimed at improving the technique of manufacturing the print head and the design thereof is underway. However, it is still difficult to provide print heads that can prevent “the banding phenomenon” perfectly because of manufacturing cost, print quality, and technique.
Such “a banding phenomenon” is known to be generated because of not only the “droplet deflection phenomenon” but also “unevenness in density” of a print head.
Specifically speaking, known print heads express shading of each color by emitting several kinds of dots including “no dot” from individual nozzles. However, part of the nozzles may print dots smaller (or larger) than that of the size corresponding to input densities (pixel values) because of production error, causing linear “unevenness in density” to generate the banding phenomenon.
Accordingly, JP-A-1-129667, JP-A-3-162977, and JP-A-5-220977 cope with the unevenness in density by actually printing a test pattern using a print head that suffers from “unevenness in density”, reading it with a scanner to form an unevenness-in-density correction table, a unit-by-unit gradation-correction table, or an average-nozzle-gradation correction table, and correcting gradations or the like on the basis of the tables.
However, with the technique of correction using the foregoing unit-by-unit gradation table (the minimum unit is a nozzle), it is difficult to scan one dot accurately in consideration of the optical characteristics of scanners, so that it is difficult to provide an accurate gradation correction table.
With the technique of correction using the unevenness-in-density correction table or the average-nozzle-gradation correction table, it is difficult to correct the density strictly. Any methods cannot ensure reduction in the unevenness in density.